calling anyone at all who's good at maths!!

Hi, I am afraid I am a complete "duffer" at maths and I need to know this: (there is a sec school related issue, I promise!!): if you were in school for 30 and a half days out of 36, what would that be as a percentage? Thank you so much.

Thank you SO much to ALL of you and thanks, Faintly, for telling me how to work it out!! I have forgotten my maths from the 70s!! I was saying to Scary the other night that I only managed a CSE grade D - shameful!!! Am good at English and Science, though, so not a complete duffer!!!! Thanks again.